This invention relates to subsynchronous resonance detection in A.C. power supply systems.
Subsynchronous resonance is a phenomenon in electrical power systems in which electrical frequencies below 60 Hz, generated by series capacitors which are used for series inductance compensation, interact with natural mechanical frequencies of a generating machine. This can result in substantial torque increases on the generating machine shafts.
The large masses of the turbines and the shafts connecting the turbines which act like springs have natural mechanical frequencies, some of which are below 60 Hz. Likewise, series capacitors in the electrical system also form electrical tuned frequencies, some of which are below 60 Hz. When transformed onto the rotor side these electrical frequencies are reflected as a different frequency. The reflected frequency can result in an unstable electro-mechanical system if it coincides with the mechanical frequency of the machine.
This unstable electro-mechanical system can be disturbed by an exciting force or a system transient which may be periodic or an impulse and can set up oscillations. Although there is generally some damping available in the electro-mechanical system, it is generally quite low and can easily be overcome by the exciting force which can set up growing oscillations. The growth of oscillations can continue to the point that the shafts become twisted and damaged.
This phenomena, known as subsynchronous resonance, occurs because of interchange and oscillation of energy between the electrical and mechanical systems, the energy being alternatively stored in series capacitors and in the various shaft sections of the generating machine.
The energy oscillations between the electrical and mechanical systems effectively modulate the normal 60 Hz voltage wave through which the power flows from the generating machine to the electrical system.
The modulation of the 60 Hz voltage is complex in nature, because the mechanism inherent in the generating machine effects modulation both in amplitude and in frequency since the amplitude of the voltage wave is a function of frequency. Furthermore, under certain conditions of subsynchronous resonance the effect is such that energy interchange reinforces the oscillations positively and the energy interchange keeps on increasing with larger energy being diverted into this oscillating system.
Prior art techniques for detecting such resonance have included filter systems to filter out normal power frequency components and thereafter treating the subsynchronous signal. Alternatively, detection is effected by sensing shaft rotation. This sensing of subsynchronous resonance by this invention is effected by an entirely different technique.